(1) Field of the Invention
More particularly the invention relates to microwave coupler devices of planar circuit form for splitting an input signal into two, equal amplitude, quadrature outputs.
(2) Description of Related Art
One such device, known as a branch arm coupler, is shown in FIG. 1 of the accompanying drawings which is a plan view of the coupler.
The coupler is formed on a thin planar substate 1 of dielectric material. On one side, the underside as shown in FIG. 1, the substrate 1 is provided with an electrically conductive coating (not shown) which serves as a ground plane. On the other side of the substrate there is an electrically conductive pattern comprising four limbs 3, 5, 7 and 9 disposed in rectangular configuration.
The input to the coupler is applied between the ground plane and a conductive strip 11 extending from one corner of the limb pattern. A dummy load 13 is connected between the ground plane and a second conductive strip 15 extending from an adjacent corner of the limb pattern. Two outputs are derived respectively from between the ground plane and third and fourth conductive strips 17 and 19 extending respectively from the other two corners of the limb pattern.
The widths and lengths of the limbs 3, 5, 7 and 9 are chosen in conjunction with the thickness and the material of the substrate so that each of one pair of opposite limbs 3 and 7 constitutes, with the ground plane, a transmission line having a desired characteristic impedance Zo and a length equal to a quarter wavelength at the centre of the frequency band over which the coupler is required to operate, and each of the other two limbs 5 and 9 constitutes with the ground plane a similar transmission line, but of characteristic impedance Zo/.sqroot.2. Similarly, each of the strips 11, 15, 17 and 19 forms with the ground plane a transmission line of characteristic impedance Zo, and the dummy load 13 has an impedance Zo.
In operation the coupler splits an applied input signal A into two equal amplitude quadrature outputs -A and -jA which appear at the outputs constituted by strip portions 17 and 19 respectively and the ground plane. Such a coupler finds application, for example, in balanced mixers and comparator circuitry.
One problem which arises in the design of such a coupler is that the gap 21 between the limbs 3, 5, 7 and 9 becomes excessively small at millimetric wave frequencies, especially if a substrate of relatively low permittivity material is used, giving rise to undesired cross-coupling and consequent degradation of performance.